This invention relates to the control of hydraulically operated elements or systems.
A particular application of the present invention is to the control of two hydraulically driven winches so that they are effectively driven in step or unison irrespective of the effect of changes in external conditions upon the wires or warps of the winches.
In the control of two separate winches used to control the shooting, towing and hauling-in of the warps associated with a trawl as is commonly used for fishing it is important to know and ensure that the trawl equipment is always maintained, as far as possible, in an optimum setting for the purposes of fishing.
It is known that in the setting of a trawl particularly those incorporating trawl boards that the pay-out of the warps should be as uniform as possible. In practice, this requires careful control of the parallel operation of the two winches usually involved so that the tension forces in the warps remain as far as possible balanced. The tension forces in the warps arise from a variety of causes including the following-the loads imposed by the trawl itself, the weight of the warps payed-out, the weather conditions, the motion of the vessel or vessels in the case of pairs fishing in which latter one warp is controlled from one vessel and the other is controlled from a second vessel.
It is known that, under conditions of travelling straight ahead and in the absence of any under water obstructions or fasteners interfering with the free passage of the trawl, that a means of achieving balanced warps is to ensure that equal amounts of warp are payed-out.
In practice, visual markers on the warps have been used to indicate to the operator the relative positions of the warps in terms of the amounts payed-out. However, reliance solely upon visual markers calls for a continuous vigilance on the part of the operator, and such vigilance is very difficult to maintain under the conditions frequently encountered at sea. Such difficulties can readily be enhanced in the case of pairs fishing.
Also it has been proposed to use devices such as extensiometers for the purposes of monitoring the instantaneous tensions in the warps, and relying upon the operator to take whatever corrective action he thinks necessary to maintain balanced tensions in the warps.
A further difficulty with operator monitoring is that whilst the tensiometers provide indications of the tensions in the warps these indications do not necessarily provide a direct indication as to whether or not the warp with the larger tension is actually the warp with an incorrect tension. Such difficulties can arise through manoeuvering and as a result of cross currents.
Turning now to the possible conditions arising when a vessel changes its heading it will be apparent that the change of heading introduces a considerable variation in the relative positioning of the vessel and the trawl. Thus, in practice, it is possible when a tight turn has to be effected for the warps to cross over so that the actual tensions produced in the warps could wholly mislead the less experienced operator.
In the case of pairs fishing similar difficulties can arise. Additional problems also arise in maintaining relative stations between vessels such as are involved in keeping the trawl in its optimum setting.
Since the load on the winch barrel or drum is reflected in the pressure conditions prevailing in the hydraulic drive circuits for the winches it has been proposed to monitor the tension conditions in the warps indirectly by monitoring the pressure in the hydraulic drive circuits of the winches. In one such known arrangement the respective pressure levels in the hydraulic circuits are monitored and any difference between the pressure conditions in the respective circuits is used to adjust the system in such manner that the difference is eliminated. The adjustment is effected by automatically pressure balancing the pressures in the hydraulic circuits of the winches by providing a pressure balancing connection between the circuits. In such a system if the pressure increases or falls in any one circuit the change is compensated by shunting fluid from the other circuit irrespective of whether or not such is the correct adjustment to make. In other words the conditions within the two circuits are closely related.